In the document “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE 802.11d-2001, Part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications”, the 802.11 standard defines a method of regulating traffic in the wireless network. That method makes use of a system of congestion windows (CW) for regulating the traffic. According to that standard, in order to determine the instant at which a data packet is to be transmitted, a station draws a random number between 0 and CW-1, the value of CW being an integer lying in the interval of values CWmin to CWmax as specified by the 802.11 standard.
The value for CW is used to launch a countdown for transmission of the packet, with the counting being deferred if the station observes that some other station is already transmitting. Unfortunately, that congestion window system leads to a large number of collisions on the wireless network that, from the point of view of a user, are equivalent to a large loss of bandwidth.
Another method known as the “tournament mechanism” is suitable for being used by the various stations in order to regulate the transmission of packets and limit the collision rate. The tournament mechanism is described in the document by Z. Abichar and M. Chang, entitled “CONTI: constant time contention resolution for WLAN access”, IFIP Networking 2005.
The tournament mechanism consists in organizing a kind of tournament between the stations that have packets for transmission. A transmission is made up of a certain number of selection rounds, each executed during a time interval of predefined duration that enables each station to have sufficient time to hear transmission by any other station. This duration is typically equal to the duration of the time interval known as the “SlotTime” that, in the IEEE 802.11 standard, is defined as the elementary time interval in the procedure for resolving contention by congestion windows.
A tournament operates as follows. At the beginning of the tournament, all of the stations having packets for transmission have the possibility of being authorized to transmit their respective packets. On each selection round, one or more stations are liable to be deleted from the list of stations that are authorized to transmit, depending on whether or not they are allocated an authorization to transmit during the selection round in question. At the end of the tournament, only those stations that have not been eliminated are authorized to transmit. If a plurality of stations are still in the running, they transmit simultaneously, thereby giving rise to a collision and thus to disturbed reception, with it being impossible to receive the transmitted data packets correctly. Those stations then need to participate in the following tournament in an attempt to transmit their packets once again.
When transmitting a data stream that requires priority access to the network or that requires a wide passband, a solution for regulating transmission that makes it possible to guarantee a certain level of performance in terms of bandwidth or data rate and also in terms of quality of service, is described in the patent document published under the No. WO 2009/095628.
That document describes a mechanism for sharing access to the network fairly between the various user stations, which mechanism makes it possible to manage coexistence between a plurality of classes of packets. That mechanism takes account of the class to which a packet belongs in order to determine how transmission authorizations or prohibitions should be allocated during a tournament. That mechanism makes it possible to control the probability with which a station transmits a packet on a class-by-class basis. In particular, it is possible to give preference to one class of packet over another, and thus to set up a method of managing priorities between classes. In that way, the method may be adapted to each class of packet depending on the needs specific to a class.
Nevertheless, the inventors have found that those implementations of the tournament mechanism do not provide sufficient performance when a given tournament involves stations that are to transmit packets at different data rates. It is entirely possible for the various stations participating in a given tournament to be implementing different standards of the 802.11 family (e.g. 802.11b, 802.11g, 802.11n, etc.). Consequently, it is possible for a first station to transmit with a mean data rate of 1 megabit per second (Mbit/s), while another station transmits with a mean data rate of 11 Mbit/s.
Given the way in which tournaments are organized, the mean data rate of the system comprising all of the stations participating in a tournament will be the lowest of the packet data rates to be transmitted by the various stations. Consequently, stations that are capable of transmitting at a rate greater than that determined for the packets to be transmitted by other stations are penalized by the presence of those other stations in the tournament.
Furthermore, for any given station, the rate with which a set of packets ought to be transmitted may vary over time. It is therefore not possible to know in advance the respective needs of the stations participating in a tournament.
There is thus a need to adapt the tournament mechanism dynamically to the data rates of the packets for transmission by the various stations.